1. Field of the Invention
The present invention relates to a color video printing method for printing a full-color image by using a frame signal reproduced from a field signal.
2. Related Art
A video image, that is, a pictorial image picked up by a video camera, is constituted of a plurality of lines. For example, video images picked up by a TV camera may be classified into frame image each constituted of 525 lines and field images each constituted of 262.5 lines, half the number of the frame image lines. The frame image is superior in quality to the field image. However, many more memory locations are necessary for recording a frame signal, that is, a video signal for one frame. This is inconvenient particularly in an electronic still camera, because the number of recordable images decreases.
Therefore, it is desirable to record an image in the form of a field signal, that is, a video signal for one field, and print the image in the form of a frame image. Because the field image is constituted of lines half the number of those of the frame image, it is necessary to double the lines by interpolation. Conventionally, an average of image data of a pair of pixels which are adjacent in the vertical direction in the field image is utilized as image data of a pixel to be interpolated between the pair of pixels, this conventional interpolation method will be hereinafter referred to as a single-directional interpolation. However, the conventional interpolation method has a problem when it is applied to such a thermal video printer as shown in FIG. 13, whose thermal head 2 has an array of heating elements 3a, 3b . . . aligned in a main scan direction (corresponding to the horizontal scanning direction of the video signal) and records images on a recording paper 5 in an area gradation method, such as disclosed in U.S. Pat. No. 5,232,294, while the recording paper 5 is moved in a sub scanning direction perpendicular to the main scanning direction relative to the thermal head 2.
In such a case, when the density of the image gradually changes, e.g., increases in the main scanning direction, jagged patterns as shown in FIG. 13 would be provided because the width of an interpolated line 6 would increase stepwise in a similar way as adjacent lines 7 and 8.
To avoid the above problem, an improved method has been known, for example, from JPA 63-187785, wherein three pairs of pixels are detected, which are disposed on opposite sides of a pixel to be interpolated and are aligned therewith in vertical and diagonal directions, respectively. Then, a difference of image data between the two pixels of each pair is determined, and is compared with one another. An average value of image data of one of the three pairs which has the smallest difference therebetween, is selected as the image data to be interpolated. This conventional interpolation method will be referred to as a conventional triple-directional interpolation.
However, according to the above-described conventional triple-directional interpolation, if all the pixels of both diagonal pairs which are disposed on diagonal opposite sides of a pixel P to be interpolated, have image data "0", image data of value "0" would be interpolated as image data of the pixel P, through the pixels of the vertically disposed pair having image data of remarkably larger values, e.g., "100" and "95", as is shown in FIG. 14. Obviously, interpolation of image data "0" in the location of the pixel P is unsuitable in the case shown in FIG. 14. Rather, it is desired interpolate the average value "97.5" of the data "100" and "95" for the pixel P. Therefore, the conventional triple-directional interpolation still has a problem in this respect.